One example of an electronic device that generates dynamic output signals is a user input device for performing operations in a computer system. Such input devices generate output signals based on user operation of the device or user data or commands entered into the device. The operations generally correspond to moving a cursor and/or making selections on a display screen. By way of example, the input devices may include buttons or keys, mice, trackballs, touch pads, joy sticks, touch screens and the like. Touch pads and touch screens (collectively “touch surfaces”) are becoming increasingly popular because of their ease and versatility of operation as well as to their declining price. Touch surfaces allow a user to make selections and move a cursor by simply touching the surface, which may be a pad or the display screen, with a finger, stylus, or the like. In general, the touch surface recognizes the touch and position of the touch and the computer system interprets the touch and thereafter performs an action based on the touch.
Touch pads are well-known and ubiquitous today in laptop computers, for example, as a means for moving a cursor on a display screen. Such touch pads typically include a touch-sensitive opaque panel which senses when an object (e.g., finger) is touching portions of the panel surface. Touch screens are also well known in the art. Various types of touch screens are described in applicant's co-pending patent application Ser. No. 10/840,862, entitled “Multipoint Touchscreen,” filed May 6, 2004, which is hereby incorporated by reference in its entirety. As noted therein, touch screens typically include a touch-sensitive panel, a controller and a software driver. The touch-sensitive panel is generally a clear panel with a touch sensitive surface. The touch-sensitive panel is positioned in front of a display screen so that the touch sensitive surface covers the viewable area of the display screen. The touch-sensitive panel registers touch events and sends these signals to the controller. The controller processes these signals and sends the data to the computer system. The software driver translates the touch events into computer events. There are several types of touch screen technologies including resistive, capacitive, infrared, surface acoustic wave, electromagnetic, near field imaging, etc. Each of these devices has advantages and disadvantages that are taken into account when designing or configuring a touch screen.
In conventional touch surface devices, and other types of input devices, there is typically an operational amplifier that amplifies the output signal of the device. The output signal is a dynamic signal in that it changes between two or more states (e.g., a “touch” or “no touch” condition). In conventional devices, the amplifier may be followed by an output signal compensation circuit that provides a compensation signal to offset an undesired portion (e.g., static portion) of the output signal. The problem with this configuration is that the amplifier amplifies both the dynamic signal of interest as well as the undesired static or offset portion.
Additionally, by compensating the output signal after it has been amplified, conventional compensation methods provide poor utilization of the output dynamic range of the amplifier, which results in poor sensitivity in detecting dynamic changes in the output signal.
Furthermore, in devices wherein the output signal is a charge waveform (e.g., an output signal from a capacitive touch surface), a relatively large feedback capacitor is typically connected between the output of the amplifier and the inverting input of the amplifier in order to accommodate relatively large charge amplitudes at the inverting input of the amplifier. The charge amplitudes should be sufficiently large to provide a sufficiently high signal-to-noise (S/N) ratio. The large feedback capacitors, however, consume a significant amount of integrated circuit (IC) chip “real estate” and hence, add significant costs and size requirements to the IC chips.
It has been found that the mutual capacitance (CSIG) across sensing nodes (a.k.a., pixels) will vary across a touch surface panel because of the geometrical and physical properties of the panel and a variety of other manufacturing tolerance related factors. Therefore, a constant compensation signal does not provide optimized compensation to every pixel output. Additionally, the touch surface electrodes can impose a phase delay on the drive or stimulus waveform passing through it. For transparent panels utilizing Indium Tin Oxide (ITO) conductive traces, for example, the resistive component may be large and varies depending on pixel location within the panel and the physical geometries of the ITO traces which connect the pixels to the sensing circuitry. For higher resistance values, a significant time (phase) delay in the propagation of the stimulus voltage through the panel may occur. This phase delay causes the output signals from the panel to be delayed before entering an amplifier, for example.